Our objective is to determine if mammotropic hormones, when acting to promote malignant transformation of mammary epithelium, act as cocarcinogens with MuMTV to alter expression of endogenous MuMTV genetic information in the mammary cell. Normal, premalignant and malignant Balb/c mammary epithelial cells are studied in vitro to facilitate kinetic measurements of virus expression and cell transformation as a function of hormone treatment. Molecular probes tailored to specific regions of the MuMTV genome, immunologic probes tailored to specific structural gene products, and quantitative markers for malignant transformation are used to: 1) identify specific changes in expression of the MuMTV genome which occur in hormone-induced malignant cells; and 2) to determine if hormonal induction of MuMTV expression in normal cells results in malignant transformation.